3AS to 5AS Guide and Remote Central Locking Tweaks

Originally written by Metropower electrical guru TonyB this guide includes wiring diagrams of how to upgrade your 3AS alarm system to the later 5AS unit.


I found that to lock/unlock (arm/disarm) my Ascot R100 etc you have to stand very (very) near to the car to use the transponder (fob). Even then a second blip may have been required.


On investigation, it was found that on the 5AS alarm, the aerial was buried in the wiring harness to the alarm. The length was only approx 110mm. Colour White/Yellow. This is very bad RF practise. You don’t bury an aerial in effectively a copper shroud.

So I removed this wire and replaced it with one 346mm long. This is half wavelength for the nominal transmit frequency of 433MHz. I dressed the wire behind the heater and cassette box held in place by tie wraps and to stop snagging etc.

The length does not have to be that accurate – say +/-10mm.

With this little modification, I can now lock/unlock (arm/disarm) the car with a range of about 20ft and can be done through my lounge window.


With respect to the 3AS alarm – this has a coaxial connection using coaxial cable. Importantly this cable is not buried in the harness. The length of this cable is just over 346mm so need not be modified. Again for slightly better response, it can be dressed, like the 5AS behind the heater and cassette box.


The anti-theft alarm ECU contains a self-diagnostic feature that tests and confirms inputs to the ECU. When in this mode, the normal functionality of the ECU is suspended.

The following procedure must be followed to enter the self-diagnostic mode. The procedure must be completed within 5 seconds.
(where referenced below the sill button refers to the internal door lock pins found at the top of the door card)

  • With driver’s door closed, press the sill button down
  • Move the ignition switch to the on, then off and then on position
  • Lift the driver’s door sill button.

Successful entry to the self-diagnostic mode is confirmed by a brief operation of the vehicle horn. The engine will become immobilised and the MFU will emit a two-tone sound to confirm that the engine is immobilised. Failure to enter the self-diagnostic mode indicates that the ECU is not receiving sill up or down or ignition switch input. A spare vehicle key will be required to test the key barrel switches.

The following inputs can be tested and confirmation of receipt of the input is confirmed by a brief flash of the LED in the instrument pack...

Sill button
Press the driver’s door sill button down, the ECU will immediately move the sill button up to unlock the doors.

Driver’s door open
Open the driver’s door to activate the switch on the ’A’ post.

Passenger and rear doors
Open the passenger and rear doors to activate the switch on the ’A’ post and the ’B/C’ posts.

Bonnet open
Open the bonnet to activate the bonnet open switch on the bulkhead.

Tail door open switch
Open the tail door to activate the boot open switch in the tail door latch.

Key barrel switches
Operate the driver’s door key barrel switch using the spare key.

The self-diagnostic mode is exited by moving the ignition switch to the off position.

1.4 PICS

The two alarm units are pictured below showing their positions under the dash

Alarm 3AS  Alarm 5AS

3AS alarm unit (left) and 5AS alarm unit (right)


I have also included a trace of transponder response taken on a Spectrum Analyser at work. This was my fob. This is basically amplitude shift keying i.e. power on power off to produce the code.
(Well --- someone maybe “vaguely interested” in this one!!)

TransponderPlot 01


The standard Rover door actuators (made by Rockwell) are known to be problematic and are expensive to replace. you can find a much cheaper version online.

The main problem is with the driver’s door actuator. This has two sensing wires back to the alarm. If the sensing is not working and the alarm does not have that earth signal from the door actuator it can put the alarm into undocumented (funny) modes and hence difficult to arm/disarm the alarm. The LED also does not flash correctly. I had one system where the central locking started pulsing – off - on - off - on etc.

Firstly disconnect the driver’s central locking door harness by the right side of the fuse box. (Quite awkward to get at.)

Referring to my drawing, the five wires in that connector are:-

  • Orange ---> pos/neg power to actuator from alarm
  • Pink ---> pos/neg power to actuator from alarm
  • Orange/Purple ---> sensing back to the alarm
  • Pink/Purple ---> sensing back to the alarm
  • Black ---> earth used by the sensing

On the female connector (car side); with a DMM, check that the black earth wire does go to earth. If there is a poor earth then check connection under the bonnet at earth bonding point by washer bottle. The earth wire is taken to this point. If that is OK then:

  • Connect an earth wire to the Pink/Purple lead. If the earth is good in that harness then a simple 5cm jumper lead can be used to that black earth wire.
  • Sit in the car and close the door.
  • Now activate the alarm using the fob. The passenger door and boot actuators should work and the alarm should correctly function arm/disarm etc and the LED should flash correctly. If all this is ok then it means that the driver’s door actuator has gone u/s.
  • If there are still problems with the central locking then disconnect, in turn, the passenger door actuator and the boot actuator and try again with all doors closed. At least now the alarm should correctly function.
  • If the driver’s door actuator is faulty, until you get a new one, leave that shorting wire in place. It will do no harm. On cars with no central locking where that Pink/Purple wire goes into the alarm a black earth wire is permanently fitted in lieu. Again see my drawing.


This schematic details the conversion from 3AS to 5AS alarm. The internal picture of the alarm shows how to wire the relay into the arm for the horn circuit.


Photos \ Diagrams (click the link below the thumbnail to see a hi-res image \ diagram)

Cctdiag Alarm3As5As 01

Cctdiag H AncilaryCcts 02

Last modified on Thursday, 18 March 2021 16:23
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